Mobile Intel Pentium 4 Processor - M and Intel 845MP/MZ Chipset Platform Design Guide

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Mobile Intel

Pentium

4 Processor-M and Intel

845MP/845MZ Chipset Platform

Design Guide 179

The implementation of a buffer is also required by the DDR. The same VREF may be used for

both MCH-M and the DDR as well.

11.7.4.2. DDR VREF Requirements

Making the same calculations for the DDR loading, results to find the max Vref load of 1 mA, a divider

is STILL NOT feasible here as the load of 1 mA causes unacceptable drop across even small Rs, which

waste power.

Table 66. MCH-M VREF Calculation

NAME Vdd Vref

PURPOSE CORE SUPPLY VOLTAGE , STATIC

I/O REFERENCE SUPPLY

VOLTAGE, STATIC

Vdd Vref = (Vdd=/-8%) / 2

VOLTAGE Nominal (V) 2.500(+/-8%) 1.250

TOLERANCE (+/-V) 0.200 0.050

Vmax(V) 2.700 1.300

Vmin(V) 2.300 1.200

Idd Iref

Section 11.7.3 5.000 0.001

Note: The DDR core is called "Vdd" =+2.5 V +/-8% (= ±0.2 V). VREF is ("Vdd"+/-8% )/2 ±50 mV. This

means that whether the 2.5 V is 8% high or low, we need to be able to divide

that voltage by 2 within a

50-mV accuracy. This basically means to use 1% resistors, or better.

Table 67. Reference Distortion Due to Load Current

R(ohms) I(A) Vdroop(V) I2.5 total=2.5v/2R(A)

1 0.001 0.001 1.25

10 0.001 0.01 0.125

100 0.001 0.1 0.0125

1000 0.001 1 0.00125

10000 0.001 10 0.000125

100000 0.001 100 1.25E-05

1000000 0.001 1000 1.25E-06

Note: As for the MCH-M, a calculation can be made for the DDR. This shows that even with the slight load of

1 mA by the DDR it is still not feasible to use a simple resistor divider. Using the max leakage specs

provided today and trying to maintain an error of less than 1%( 12.5 mV) one needs to decrease the

resistor values such that the current just to source the divider becomes unacceptable. A divider alone

does not become an acceptable solution until current requirements are in the 100-µA range. Today, it is

not possible to guarantee this type of current requirement for these applications. Hence, the use of a

buffer is highly recommended for these reference voltage requirements.